New boundaries: Experiments verify ion behavior in plasmas

Since 1949, scientists have assumed that ions in a plasma must achieve a velocity known as the ion-sound, or Bohm, velocity before they flow out of the plasma. Back then, academic and theoretical physicist David Bohm showed that the ions must drift at a certain velocity based on electron temperature and ion mass-even if the ions have a zero temperature in the bulk plasma.

But what scientists didn't know is how the ions sped up. Bohm suggested it was the result of weak electric fields that formed in the plasma away from the thin boundary, or sheath, at the plasma's edge. Normally, that sheath is positively charged, while the rest of the plasma is electrically neutral.

In the theoretical literature describing the phenomenon, says Professor Noah Hershkowitz, the region in which ions are accelerated is often called the presheath.

Although determining the details of the plasma potential profile near boundaries is one issue critical to basic understanding of confined plasmas, until recently, no one had experimentally verified Bohm's theory and subsequent models of presheaths, says Hershkowitz. "For a long time, I've been interested in the question of if you have a plasma, and if you have various kinds of geometries and various kinds of sources, how does the plasma potential get from one place in the plasma to the other," he says.

He and graduate student Lutfi Oksuz recently concluded long-awaited experiments that verified K.U. Riemann's more recent theory about weakly collisional plasmas, in which ions collide with the neutral species. As their primary diagnostic of plasma potential, they used hot-wire emissive probes, which only emit electrons if they are biased below the plasma potential. The two determined that a weakly collisional plasma's potential profile consists of a presheath, where ions accelerate, a transition region of the sheath in which most plasma electrons are reflected, and an electron-free region of the sheath. The two published their results in the Sept. 30, 2002, Physical Review Letters.

The evidence holds significance for a number of applications, says Hershkowitz. In semiconductor processing, ions falling through the sheath pick up a lot of velocity as they travel through the sheath. Collisions in the presheath are the source of velocity perpendicular to this direction. "And since the parallel velocity is much larger than the perpendicular, you can etch vertical structures," he says. "So the presheath ion-scattering provides the ultimate limitation on how vertical structures can be etched."

In a fusion device, loss of plasma at a limiter or diverter determines both the heating and particle recirculation at that point and provides basic limits as to how much current researchers can extract, what density level they can achieve, how long materials last, and more. "The details of what happens near the sheath can determine what happens to a satellite in space: what the electric fields near the satellite look like, when particles are emitted by photoemission or by interaction with the surroundings, how that plasma is lost and so on," he says. "Understanding what's going on is an important thing to do."

Having verified the ions' speed at the sheath and determined the plasma's potential profile, Visiting Professor Greg Severn, Hershkowitz and graduate students Eunsuk Ko and Xu Wang conducted a second experiment with more than one ion species in the plasma. In that case, researchers assumed each ion species would fall out at its own Bohm velocity, he says. Experimenting with a plasma made up of equal amounts of argon and helium ions, the group learned that the heavier argon ions fell out faster at the boundary than they would have if they had been the only ion species in the plasma.

The group, which published the results in the April 11, 2003, issue of the Physical Review Letters, used laser-induced fluorescence (LIF) with a diode laser to scan the ion velocity and determine the ion drift. Hershkowitz hopes to begin a second phase of the experiment in which he and his students will add another ion species such as xenon and use LIF to determine the velocities of both species.

Hershkowitz says both experiments dispelled long-standing assumptions that weren't true. "It's a question that we felt should be answered and 50 years is a long time to not know that something is really correct or not correct," he says.